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Camouflage is an essential part of combat— whether it be camouflaged gear for troops on the ground or smoky flares in the skies to deflect heat-seeking missiles. Historically, weapons of war were rarely designed with health or environmental effects in mind. Now, a pair of scientists is touting a compound called phosphorus nitride as a safe and effective alternative to red phosphorus, which is used in many kinds of obscurants, such as smokescreen grenades and decoy flares.

A French Air Force soldier watches from a Caracal military helicopter decoy flares being fired during the Salamandre joint French Air Force and US Air Force Combat, Search and Rescue Training exercise (CSAR) at the Cazaux airbase in La Teste-de-Buch, on November 9, 2016. (Photo: MEHDI FEDOUACH/AFP/Getty Images)

What exactly does an obscurant do? Think about fog, mist or dust—each of these are small particles of something suspended in the air, and they have the effect of blocking or weakening visibility. Scientists—and the military—learned long ago that certain chemicals can do an even better job of cloaking. Early in our nation’s history, the black smoke from gunpowder made it difficult to see on battlefields. Today things are more sophisticated. Obscurants can be tailored to the specific part of the electromagnetic spectrum you’re looking to conceal. Want to block the infrared radiation coming off that tank? There’s an obscurant for that. How about defeating radar systems, or blocking night-observation sites? You get the idea.

Red phosphorus is a form of the element phosphorus. It is a more familiar substance than you may think. Besides being used in military decoy flares and fireworks, it’s also found in the striking surface of a household matchbox.

(Photo: Shutterstock)

For military applications, however, red phosphorus leaves a few things to be desired. When it's burned it produces aerosols of phosphoric acid. This acidic smoke, while not as toxic as some other smokescreens, irritates the respiratory tract. A 1997 report by the National Research Council set recommended exposure limits for red phosphorus smoke and called for the Army to study health effects further in personnel who conduct training exercises, who are likely to see repeated exposure to this kind of smoke.

More alarming, perhaps, than red phosphorus’s health effects is its reactivity. Red phosphorus ignites if it encounters friction (remember that matchbox?) or an impact when it’s being shipped or handled, even outside of a combat situation. It reacts violently with oxidizing agents, such as potassium chlorate, which work to keep a flare’s hot flames thriving. And if red phosphorus gets exposed to moisture, it can break down and release a toxic gas called phosphine. Manufacturers mitigate these problems with fillers and binders, absorbing pads and gas-venting mechanisms, but these are far from ideal fixes.

Enter chemists Ernst-Christian Koch, from German energetic materials consultancy Lutradyn, and Stanisław Cudziło, from Poland’s Military University of Technology. The pair investigated phosphorus(V) nitride (P3N5), a compound that is already known and available on the market, as an alternative to red phosphorus. They reported their results in the journal Angewandte Chemie.

First, the duo combined phosphorus nitride with a few common oxidizing agents to see whether the formulations would hold up against impact and friction. They used something called, I kid you not, the BAM test. The acronym comes from the name of the German institute that developed the machine used in the test- Bundesanstalt für Materialforschung und -prüfung, which translates to Federal Institute for Materials Research and Testing. Somehow I think knowing that ruins the fun a little.

Back to the research. With just one exception, the formulations of phosphorus nitride were not at all sensitive to friction and only mildly sensitive to impact. In further tests, Koch and Cudziło found that a certain phosphorus nitride formulation burned far less sluggishly than the same formulation containing red phosphorus. They also demonstrated that phosphorus nitride does not break down to emit toxic phosphine gas.

Is this the perfect solution? Has red phosphorus met its match for military applications? Maybe. I would want to see tests carried out on real-world scales where it’s likelier that mixing of the formulations isn’t quite as perfect. (These tests were carried out on small scales, fractions of ounces of material.) Also, this proposed alternative still produces a phosphoric acid smog when it ignites. At the end of the day, decisions about whether to replace red phosphorus will depend on the magnitude of that health risk as compared to the other problems that phosphorus nitride solves.

That balancing act is going to take more tests to sort out. Angewandte Chemie’s press release helpfully points out that “phosphorus(V) nitride as a commercial compound has no large-scale application yet.” So it looks like munitions researchers might want to at least start buying some.